What kind of disk device drivers are you using inside the virtual machines (virtio/vda or ide/hda?)
I've seen this happen before too, that basically idle kvm vm's generate I/O load on the hypervisor, not quite sure why.,

Steve Timm


-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]] On Behalf Of Chuck Munro
Sent: Monday, January 21, 2013 2:15 PM
To: scientific-linux-users
Subject: SL-6 KVM Disk Activity

Hello All,

I'm very curious (and a bit concerned) about constant, once-per-second disk accesses when running KVM virtual OS's on three SL-6.3 VM host machines.

Whenever I start one or more VM OS's, I see disk write activity every second, starting the instant I launch the VM.  With quiescent VMs, each instance of libvirtd averages about 15.5 K/s writes, and no reads.  This is with BSD- and Linux-based virtual machine instances.

All virtual machines have more than enough RAM assigned so there should be no swapping activity.  'top' on any VM shows zero swap in use, but 'iotop' on the host servers shows constant disk writes for each instance of libvirtd.  This happens even when a virtual machine should be doing no disk accesses at all.  The date stamp on each .qcow2 or .img file is constantly being updated, so that's where the I/O is going.

Is this normal?  My main concern is wear and tear on the disks.

Any advice on how to reduce this activity?

Chuck